Cell adhesion is a complex process that is important for maintaining tissue integrity and generating physical and permeability barriers within the body. Cell adhesion is mediated by specific cell surface adhesion molecules (CAMs). There are many different families of CAMs, including the immunoglobulin, integrin, selectin and cadherin superfamilies, and each cell type expresses a unique combination of these molecules.
Although cell adhesion is required for certain normal physiological functions including wound repair, there are situations in which cell adhesion is undesirable. For example, many pathologies, such as metastasis, autoimmune diseases, and inflammatory diseases, involve abnormal cellular adhesion. Cell adhesion may also play a role in graft rejection. In such pathologies, modulation of cell adhesion may be desirable.
Multiple Myeloma and Treatment Resistance
Multiple myeloma is an incurable malignancy of the plasma cell characterized by migration and localization to the bone marrow where cells then disseminate and facilitate the formation of bone lesions. Despite initial responses to chemotherapy, myeloma patients ultimately develop drug resistance and become unresponsive to a wide spectrum of anticancer agents, a phenomenon known as multidrug resistance (MDR). The development of resistance to front-line chemotherapeutic drugs, such as melphalan (an alkylating agent) and doxorubicin (an anthracycline), is a major factor responsible for treatment failure; thus, the median survival of 3.5 years has remained largely unchanged for the past three decades.
There are many potential mechanisms for drug resistance in myeloma including reduction in intracellular drug accumulation due to overexpression of MDR1/Pglycoprotein, alterations in drug targets such as topoisomerase II, enhanced DNA repair, and overexpression of anti-apoptotic proteins such as bcl-2. These mechanisms are chiefly due to alterations in the malignant cell itself and can be studied using in vitro human myeloma cell. However, these mechanisms alone cannot account for all drug resistance, nor are they likely to explain cell survival following initial cytotoxic drug exposure. Additional mechanisms conferring low level drug resistance are believed to play important roles in the survival and expansion of the malignant cell population. Recent studies have demonstrated that certain resistance mechanisms are observed only in vivo, suggesting that interactions between malignant cells and the surrounding microenvironment may be important in determining response to chemotherapeutic drugs. Factors that allow for tumor cell survival following initial drug exposure need to be identified because these factors may eventually allow for expression of genes associated with acquired drug resistance. In addition, through understanding the mechanism that suppresses drug-induced apoptosis as well as other mediators of drug resistance, improved therapies can be developed which interfere with, or inhibit the resistance.
It is known that intercellular interactions can contribute to tumor cell survival during exposure to cytotoxic stresses such as radiation. It is also known that certain resistance mechanisms may only be functional in vivo, where tumor cells continue to interact with environmental factors such as extracellular matrix (ECM) and cellular counter-receptors. For example, Teicher et al. showed that mammary tumors made resistant to alkylating agents in vivo are sensitized to cytotoxic drugs once removed from the animal, Tumor resistance to alkylating agents conferred by mechanisms operative only in vivo, Science 247:1457,1990. Adhesive interactions between same cell types are known to confer resistance to alkylating agents via alterations in cyclin dependent kinase inhibitors such as p27kipl, although the cell surface molecules mediating this type of kinetic resistance have yet to be identified. In addition, adhesion to ECM has been reported to induce P-glycoprotein expression and confer doxorubicin resistance in rat hepatocytes.
The integrin family of cellular adhesion molecules is a major class of receptors through which cells interact with extracellular matrix components (ECM). Recent evidence has implicated the integrins as being closely involved in the pathology of many diseases. Integrins have been shown to participate in intracellular signal transduction pathways that may contribute to tumor cell growth and survival. Experimental evidence has implicated the PI integrins and fibronectin as playing a part in apoptotic suppression and cell survival. For example, Zhang et al. has demonstrated that fibronectin adhesion through α5β1, (VLA-5) prevents cells from undergoing serum-starvation induced apoptosis by upregulating Bcl-2. The alpha 5 beta I integrin supports survival of cells on fibronectin and up-regulates Bcl-2 expression, Proc Natl Acad Sci USA 92(13):6161, 1995.
Similar observations are made by Scott et al. and Rozzo et al., who found that anti-β1 antibodies and antisense oligonucleotides, respectively, enhanced chromatin condensation and nucleosomal DNA laddering, characteristics of cells committed to apoptosis. Fibronectin suppresses apoptosis in normal human melanocytes through an integrin-dependent mechanism, J Invest Dennatol 108: 147, 1997. Induction of apoptosis in human neuroblastoma cells by abrogation of integrin mediated cell adhesion, Int J Cancer 70:688, 1997. PI integrin activation through interactions with ECM components such as fibronectin directly decreases DNA strand breaks in tumor derived endothelial cells exposed to a number of DNA damaging agents, including etoposide and ionizing radiation.
The α4β1 (Very Late Activation Antigen 4, or VLA-4), α5β1 (VLA-5), and α4β7 heterodimers are the major fibronectin receptors of the integrin family. Although VLA-5 and α4β7 expression are variable in most B cells during malignancy, VLA-4 is strongly expressed in myeloma cells collected from bone marrow. VLA-4 is unique among the integrins as it is the only heterodimer to have been shown to mediate cell-ECM as well as cell-cell interactions. VLA-4 binds to the CS-1 region of fibronectin as well as to vascular cell adhesion molecule-1 (VCAM-1) via separate binding sites. Adhesion to fibronectin via VLA-4 has been shown to prolong eosinophil survival and to downregulate FAS antigen expression, leading to a decrease in cell death. In early hematopoietic and germinal center B cells, adhesion to fibronectin or VCAM-1 via VLA-4 suppresses the apoptotic pathway and contributes to positive selection.
As myeloma cells adhere in the bone marrow, they stimulate their own growth and cause osteoclast formation through the increased synthesis and secretion of cytokines such as IL-1β, TNF-β, M-CSF, and IL-6. IL-6, a potent growth factor for myeloma cells, is secreted from both tumor and stromal cells in response to co-adhesion and VLA-4 ligation. VLA4 associates with or causes the phosphorylation of a number of signal transduction molecules, including CD19 receptor-associated protein tyrosine kinases and focal adhesion kinase (pp125FAK, or FAK), which is an upstream activator of mitogen activated protein kinase (MAPK), among other proteins. FAK plays a major role in suppressing apoptosis both in adherent and suspension cells, and its cleavage by caspases early in the apoptotic process further emphasizes its importance within the cell.
Peptide and Cell-Adhesion
Peptides capable of modulating cell adhesion have been reported. U.S. Pat. No. 6,169,071 to Blaschuk, et al. discloses cyclic peptides comprising a cadherin cell adhesion recognition sequence HAV and methods for modulating cadherin-mediated cell adhesion in a variety of contexts. U.S. Pat. No. 6,020,460 to Pierschbacher, et al. disclosed conformationally stabilized synthetic Arg-Gly-Asp-containing peptides which have increased affinity and selectivity for the vitronectin receptor over that of linear, Arg-Gly-Asp-containing synthetic peptides. Such peptides are used to modulate cell-adhesion.
However, none of the above cited art references discloses or even suggests the administration of peptides to enhance chemo- or radiotherapy, nor in situations when such drugs may help to overcome cell adhesion inhibition-mediated drug resistance. In contrast, and teaching away from the instant application, the rise of drug resistant tumor cells is usually associated with reduced migratory and invasive ability and a lower adhesion capacity (see abstract by Scotlandi K. et al., Multidrug resistance and malignancy in human osteosarcoma, Cancer Res 1996 May 15;56(10):2434–9).
Accordingly, there is a need in the art for compounds that modulate cell adhesion to improve, for example, the efficacy of chemotherapy and radiation therapy of cancer cells such as multiple myeloma without the aforementioned disadvantages. The present invention fulfills this need and further provides other related advantages that will become apparent to one of ordinary skill of the art upon reading the following disclosure.